1. Field of the Invention
The invention relates in general to an organic light emitting diode (OLED) display, and more particularly to a method for driving OLED pixels.
2. Description of the Related Art
In a pixel of an organic light emitting (OLED) display, charges are stored in a storage capacitor for controlling the luminance of an OLED via a thin film transistor (TFT). Referring to FIG. 1, a schematic diagram of a conventional pixel circuit is shown. The pixel circuit 100 includes an N-type TFT 102, a storage capacitor 104 and an OLED 106. The two ends of the storage capacitor 104 are respectively coupled to the gate G and the source S of the TFT 102. The voltage drop of the storage capacitor 104 is denoted by Vgs. The positive end of the OLED 106 is coupled to the source S of the TFT 102, whose voltage level is denoted by VOLED. The current flowing by the TFT 102 is controlled by the voltage drop Vgs, that is, the current IOLED of the OLED 106 is equal to K*(Vgs−VTH)2. The voltage drop Vgs is a voltage difference between a pixel voltage Vdata and the voltage level VOLED at the positive end of the OLED 106. Therefore, the luminance of the OLED 106 can be controlled by adjusting the pixel voltage Vdata.
However, when the above-mentioned TFT 102 is operated, a shift of the threshold voltage occurs on the TFT 102. The voltage shift amount is related to a manufacturing process, operation time and current of the TFT 102. Therefore, in terms of all pixels on the display panel, due to difference of the pixels in the operation time, conductive current and manufacturing process, the shift amount of the threshold voltage of each pixel is different, which in turn causes the luminance and the received pixel voltage of each pixel to have different corresponding relationship. Therefore, the issue of non-uniform frame luminance occurs.
Besides, the OLED 106 has an increasing voltage drop, that is, an increasing VOLED, along with the using time. Referring to FIG. 2, a characteristic diagram of the OLED 106 is shown. From FIG. 2, it can be seen that the OLED 106 has an increasing VOLED under a long using time. Therefore, the conductive current IOLED is reduced under the long using time according to the equation Vgs=Vdata−VOLED. The decreasing current IOLED causes that the pixel voltage Vdata cannot drive the OLED 106 to reach the predetermined luminance. That is, the overall luminance of a display frame is reduced.
In conclusion, even if the issue that the threshold voltage shift causes unequal driving currents to flow by the OLEDs can be solved, frame luminance is still reduced or becomes non-uniform due to material feature variation of the OLEDs. Therefore, how to simultaneously solve the issue of TFT threshold-voltage shift and OLED material decay is indeed an essential subject for the relevant industrials.